Wherever a sensor is used to measure a variable such as temperature, it is usually desirable to develop a sensor output signal which is accurate and which can be read with a reasonably high degree of resolution. Unfortunately, accuracy and resolution are not easily obtained simultaneously.
For example, in applications in which a sensor output signal must be digitized by an A/D (analog to digital) converter, it is frequently necessary to amplify the sensor's output signal in order to make full use of the range of the A/D converter. Such amplification improves resolution, but tolerances in the amplifier and its associated circuitry cause errors in the digitized sensor reading. These errors can include offset voltage errors, gain errors, and error components resulting from the cross-products of offset and gain errors. Such errors are illustrated in FIG. 1 which graphs the output voltage of a hypothetical temperature sensor.
In FIG. 1, the line 10 illustrates an ideal, linear sensor output signal. Line 12 depicts a sensor output signal which is offset from the origin of the graph and which is said, therefore, to have an offset error. Line 14 represents a sensor output signal having a gain error insofar as its amplitude differs from the output of the signal depicted by line 10. Line 16 illustrates a sensor output signal which includes both a gain error and an offset error, and is typical of the signal developed by a sensor and its associated amplifier circuitry. To identify and compensate for such errors is possible and has been accomplished with some degree of success. However, the gain and offset errors give rise to error components referred to herein (and discussed in more detail below) as cross-product errors which cannot be readily isolated and which are difficult to fully compensate for.
In some applications, a plurality of sensors are required, and the output of each must be amplified and otherwise processed. Each such sensor and its associated circuitry is considered as a separate sensor channel. This need for plural channels gives rise to yet another possibility for error; namely, the channel-to-channel error that results when the various channels do not precisely track with each other.
The foregoing discussion illustrates the need for an improved sensor circuit, and particularly for a sensor circuit which compensates for the errors discussed above.